The present invention relates in general to cardiac stimulation devices, such as pacemakers, defibrillators, cardioverters, implantable cardioverter-defibrillators (xe2x80x9cICDsxe2x80x9d), and similar cardiac stimulation devices that are capable of monitoring and detecting electrical activities and events within the heart. In particular, this invention pertains to a system and method for automatically adjusting the energy of the stimulation pulse so as to automatically set the threshold safety margin based on a series of threshold tests for optimal energy expenditure.
Implantable pacemakers generate electrical stimulation pulses and deliver such stimulation pulses to atrial and/or ventricular muscle tissue of a patient""s heart at a prescribed rate and/or rhythm when, through disease or other causes, the heart is not able to maintain the prescribed heart rate or rhythm on its own. When the delivered electrical stimuli are of sufficient energy, they cause the cardiac muscle tissue to depolarize, and therefore contract, thereby forcing the heart rate or rhythm to track the delivery of the electrical stimuli. When the delivered electrical stimuli are of insufficient energy, depolarization does not occur, and the heart rate or rhythm is not controlled by the pacemaker. Hence, for the pacemaker to perform its intended function, it is critically important that the delivered electrical stimuli be of sufficient energy to depolarize the cardiac tissue, a condition known as xe2x80x9ccapturexe2x80x9d.
The energy of the electrical stimuli generated by an implanted pacemaker is derived from the energy stored in the pacemaker power source or battery. The pacemaker battery has a limited amount of energy stored therein, and the generation of electrical stimuli represents by far the greatest drain of such energy. In order to preserve this limited energy and to prolong the life of the battery, it is known in the art to adjust the energy of the delivered electrical stimuli so that it is just sufficient to cause capture, with an appropriate safety margin.
Capture detection may occur on a beat-by-beat basis or on a sampled basis. In one embodiment, a capture threshold search is performed once a day during at least the acute phase (e.g. the first 30 days) and less frequently thereafter. A capture threshold search would begin at a desired starting point (either a high energy level or the level at which capture is currently occurring) and decrease the energy level until capture is lost. The value at which capture is lost is known as the capture threshold. Thereafter, a safety margin is added to the capture threshold. For a more detailed description of capture and the implementation of capture detection circuitry and algorithms refer, for example, to U.S. Pat. No. 4,729,376 (Decote, Jr.); U.S. Pat. No. 4,708,142 (Decote, Jr.); U.S. Pat. No. 4,686,988 (Sholder); U.S. Pat. No. 4,969,467 (Callaghan et. al); and U.S. Pat. No. 5,350,410 (Kleks et. al), all of which patents are hereby incorporated herein by reference.
The amount of energy needed to effectuate capture is known as the capture xe2x80x9cthresholdxe2x80x9d, and electrical stimuli of energy less than the capture threshold do not bring about capture, while electrical stimuli of energy greater than the capture threshold do bring about capture. By adjusting the energy of the electrical stimuli so that it is always greater than the capture threshold, but not too much greater, the limited energy of the pacemaker battery may thus be preserved. The battery energy is preserved because: (1) electrical stimuli of insufficient energy to cause capture (electrical stimuli below threshold), which stimuli represent wasted energy, are rarely generated; and (2) electrical stimuli of excessive energy (energy much greater than the capture threshold), which excess energy not only represents wasted energy, but also energy that may disadvantageously cause pectoral stimulation and/or sensation, are also rarely generated.
In general, capture verification has been regarded as a process that must be carried out continually, and conventional methods teach that capture must be verified with each and every stimulus so that a servomechanism within the pacemaker can provide a backup pulse in the event that a first pulse fails to provide capture and so that the energy of the next stimulation pulse can be adjusted upward. Reference is made to U.S. Pat. Nos. 4,969,467; 4,969,462; and 4,955,376.
However, capture verification requires a significant amount of processing time and corresponding battery current to be expended. An important aspect of conventional capture verification is setting the safety margin (also referred to as safety factor) is normally set to a fixed value. The safety margin value is determined largely by limitations in the hardware platforms that allow programmed amplitude steps of, for example, 0.125 Volt.
In ventricular auto capture systems, the safety margin has proven to be adequate since beat-by-beat capture verification is provided by the pacemaker. If a threshold increase occurs, the pacemaker responds by increasing the output amplitude, and re-finding the capture threshold, where the safety margin is applied. This method reduces the energy consumption by adding the lowest possible safety margin the pacemaker allows. Reference is made to U.S. Pat. No. 5,766,229 to Bornzin, which is incorporated herein by reference.
However, beat by beat capture verification may not be provided in atrial automatic threshold determination, resulting in a need for higher safety margins. Also, threshold variability has been demonstrated to be greater sometimes due for example to circadian variations, sinus rate variability and other factors that are present in the atrium but not necessarily in the ventricle. The problem that remains heretofore unresolved is to choose the safety margin that guarantees capture and at the same time provides adequate energy savings in an atrial and ventricular automatic threshold testing device.
In addition, conventional pulse energy is set based on the result of a single threshold test and a certain fixed safety margin. As a result, threshold testing must be repeated often to adjust the safety margin. This conventional approach may result in xe2x80x9cunder-samplingxe2x80x9d, which can lead to erroneous conclusions and an unstable pacing system. Under-sampling the threshold in turn requires frequent threshold tests, causing additional inefficiencies of the pacing system.
In view of the above, it is evident that there is still an unsatisfied need for an automatic threshold testing method that automatically adjusts the atrial and ventricular threshold safety margins and that minimizes the expenditure of battery current.
The present invention addresses the above and other needs by providing an implantable pacemaker wherein an automatic threshold testing is performed to generate a statistical model. The statistical analysis of a series of threshold tests is then used to adjust the safety margin based on statistically determined threshold and safety margin such that the possibility of a loss of capture episode is minimized.
The threshold testing in accordance with the present invention occurs when a defined trigger event initiates a testing algorithm. Such testing, because it is performed on a limited sampled basis, advantageously limits the time and power needed by the electronic circuitry (typically a microprocessor) to perform the tests, thereby conserving power and freeing up the circuitry for other processes.
Thus, in accordance with one aspect of the invention, the expenditure of battery current in the pacemaker is minimized because threshold tests are performed only on a limited sampled basis, and not with every capture verification test as is traditionally practiced in the art. Periodic threshold tests can therefore be performed less often.
A further aspect of the invention provides for adjustment of the pulse energy based on the statistical analysis of a minimal sample size of threshold tests. A pulse energy based on such a statistical analysis will provide greater confidence that the stimulation pulse energy is equal to or greater than the threshold, thereby minimizing the possibility of a loss of capture event. It is thus seen that the present invention reduces the required frequency of capture verification since greater confidence of capture exists.
In addition, the present invention addresses the problem of potential risk of non-capture by setting the stimulating pulse energy to a value that improves the confidence that capture will occur with every stimulation pulse, thus justifying the use of less frequent capture verification.
The present invention addresses the problem of under-sampling that results from the determination of the pulse energy based on a single threshold test, that requires frequent or over-sampling of capture verification by determining the pulse energy from a series of threshold tests over time, and by setting the threshold and safety margin to levels that minimize the chance that capture will be lost.
The threshold testing algorithm of the present invention includes: setting a specified number of threshold tests that will occur over a specified period of time; performing the threshold tests at the specified points in time and storing the threshold test result; calculating descriptive statistical data for the stored threshold test result; and adjusting the stimulating pulse energy based on the variability of the threshold test results.
It is still a further feature of the invention to provide an implantable pacemaker wherein once the desired number of threshold tests have been performed the stimulation pulse energy is set to value that is the lowest possible value, based on the statistical analysis of the threshold tests, that predicts, with a desired degree of confidence, that capture will occur with every stimulation pulse.